Communication systems are designed to transfer information between two devices over a medium in the presence of disturbing influences. Intersymbol interference (ISI) is one well-known disturbing influence in which transmitted symbols become elongated and interfere with adjacently transmitted symbols. This spreading or “smearing” of symbols is generally caused by the dispersive nature of common communication mediums. Because ISI has the same effect as noise, communication is made less reliable.
One of the most basic solutions for mitigating the effects of ISI is slowing down the speed at which symbols are transmitted over a medium. More specifically, the transmission speed can be slowed down such that a symbol is only transmitted after allowing previously transmitted symbol pulses to dissipate. The time it takes for a symbol pulse to dissipate is called delay spread, whereas the original time of the symbol pulse (including any time before the next symbol pulse is transmitted) is called the symbol time. No ISI will occur if the delay spread is less than or equal to the symbol time.
Although slowing down the symbol rate can eliminate the effects of ISI, it is generally an unacceptable solution for many of today's communication applications. In fact, many of today's communication applications require speeds in the multi-gigabit per second range. At such high speeds, ISI can completely overwhelm a signal transmitted over a few inches of printed circuit board trace, a few feet of copper cable, or a few tens of meters of multimode optical fiber.
Backplane Ethernet is one type of communication application that suffers from ISI. Backplane Ethernet has been standardized, at least to some degree, under the IEEE 802.3ap standard and is commonly used in devices such as routers, switches, and blade servers. The IEEE 802.3ap standard specifically defines operation characteristics for 10 Gbps Backplane Ethernet in configurations that communicate data differentially over two pairs of copper traces on printed circuit boards. Data is transmitted over each pair of copper traces in one direction using a one-bit, pulse amplitude modulation (PAM) scheme and a baud rate of 10.3125 GHz.
Although the conventional 10 Gbps speed limit already presents significant signal impairment challenges from ISI and other noise sources, providers of Backplane Ethernet applications are pushing designers to achieve speeds up to, and beyond, twice the conventional 10 Gbps limit. However, at those speeds, traditional, cost effective transceiver designs for Backplane Ethernet applications cannot reliably transmit and receive information over copper traces on a printed circuit board due to ISI and other noise sources.
Therefore, what is needed is a cost effective system for supporting backplane applications having speeds up to, and beyond, twice the conventional 10 Gbps limit.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.